This invention relates to method and apparatus for processing signals and more particularly, for detecting the transition points between the binary levels of signals.
Signals having transitions between binary levels are generated from many different devices. One device which generates such digital signals is described in the above-referenced application. In that application, Universal Product Code (UPC) symbols are optically scanned and detected to produce a video digital signal.
Codes like the Universal Produce Code are generally characterized as bar codes and consist of a series of parallel light and dark rectangular areas of different widths. In one Universal Product Code, each character is represented by two dark bars and two light spaces. Different widths are selected for the bars and spaces in order to define different characters.
Packages are labeled using the UPC characters and electrooptical scanning apparatus scans the packages to detect the coded characters. The apparatus which scans the characters detects reflected light from the characters. The reflected light is incident upon a photo-multiplier tube or other optical detector. The detector generates an electrical signal which has one level for dark bars and a second level for light spaces. Positive-going transitions and negative-going transitions occur in the electrical signals to signify transitions between bars and spaces. The transitions in the electrical signals are not perfectly sharp, but rather, tend to be spread out so that it is difficult to determine the exact instant at which the transition has occurred.
As a practical matter, in many instances it is not possible to produce signals with precisely-defined transitions. Therefore, signals are further processed to determine the point at which transitions occur.
Various circuits are known for carrying out such signal processing. These signal processing circuits typically include filters for removing unwanted components. Also, threshold devices have been employed for rejecting components which do not exceed a predetermined level. Although many signal processing techniques have been employed, a need still exists for improved signal processors which provide an ability to precisely detect transitions in such signals.
In view of the above background, it is an objective of the present invention to provide an improved signal processor which accurately and precisely determines the time of transitions between binary levels of encoded analog signals.